The Forms of Storage Used

The information store in the Mark II machine consists of the magnetic store and the electronic store. The information in the magnetic store is of considerable volume viz. 655360 binary digits: in other words it corresponds to paper on which is written 655360 digits each of which might be either 0 or 1. But this information is not particularly readily available. It is (to maintain the analogy) as if it were written in a book. In order to find any required piece of information it is necessary to open the book at the appropriate page. The electronic store has a considerable smaller capacity viz. 20480 digits but this information is much more readily available and is to be compared rather to a number of sheets of paper exposed to the light on a table, so that any particular word or symbol becomes visible as soon as the eye focuses on it.

The information in the magnetic store consists of magnetised areas of nickel on the cylindrical surface of a rotating wheel. Each digit stored is represented by one magnetised area. These 655360 areas are arranged in 256 tracks of 2560 digits each. The centres of the areas forming one track lie in a plane perpendicular to the axis of the wheel (and therefore on a circle). The 256 planes are equidistant; the 2560 digit areas on one circle are not however equidistant. The information in one track is further subdivided into two equal parts, which may be described as the left page and the right page if we continue to follow the simile of the book.

The information in the electronic store consists of 20480 digits stored in 16 `tubes' of 1280 digits each. [In fact, all sixteen tubes were not expected to be available; see p. .] A tube thus contains the same amount of information as a half-track or page of magnetic store. It may also be described as an `electronic page'. Its geometrical arrangement is however very different. A tube of information is divided into 64 `lines' of 20 digits each. These digits are stored as charges on the inner surface of the front of a cathode ray tube, the digits of one line forming a straight horizontal segment. Another line is stored in the continuation of this segment. The other lines are in parallel segments, the whole forming a rectangular array. (See photograph [omitted from this typescript]). The lines may be numbered consecutively through the 16 tubes. They could be numbered 0, 1, ..., 1023. However, this numbering is seldom used. One prefers to use the labelling obtained by transforming to the scale of 32 i.e. to teleprinter code, thus the lines are known as //, E/, ..., ££. In their geometrical arrangement on the tubes they are as below

Tube 0	Tube 1		Tube 15
// /E	/@ /A		/V /£
E/ EE	E@ EA		EV E£
@/ @E	@@ @A	......	@V @£
. . . . . .	. . . . . .		. . . . . .
£/ £E	£@ £A		£V ££

Each tube may naturally be described as formed of two `columns'. Thus the second character describing a line gives the column in which it is to be found. It might appear at first sight as if the arabic labelling of the lines would be simpler, but this is not so. The names used above for the lines have to be used in the instructions (Fig. E below) and in every other automatic connection. It is therefore desirable that the programmer should use them also.

Associated with each line is a `line pair' or `long line' consisting of that line together with the next line, e.g. the lines R/ and J/ together form a line pair and so do BH and GH. The programmer is recommended to use the former in preference to the latter. (As yet of course he is in no position to use either.) The two long lines referred to have the names R/ and BH. There is an exception to the rule that a line pair always consists of the line named together with the next. A line pair never consists of parts from different tubes. When the line named is the last of a tube the pair consists of the last of the tube followed by the so called `sixty-fifth line' of the tube. On account of this and other reasons one is recommended to use even-numbered line pairs in preference to odd-numbered ones, especially where consecutive sequences of lines are involved.

Although the information on the magnetic wheel is arranged geometrically so differently from that in the electronic store it may be found convenient to imagine it as if it were similarly arranged. There is very little to interfere with the illusion. The only convenient method for making the content of a track visible (`opening the book') is to copy the track onto a pair of tubes, a process which effectively conceals the true arrangement of the digits of the track. This way of thinking permits us to divide up the magnetic information also into lines or line-pairs. There is however a good deal of ambiguity about the naming of these lines, for a page of magnetic information could be copied onto any one of the 16 electronic tubes. We make no attempt to remove this ambiguity and accept that there are 16 alternative names for a line stored on the wheel, e.g. the same line could also be known as track 14 (left) I/ or track 14 (left) IT. There is however a presumption that if it is known by the latter name then in its principal application track 14 (left) will be copied onto tube 8 [3 in the original].

On each tube there is an additional `identification' line, sometimes known as the `sixty-fifth line'. This line does not share in the normal activities of store-lines. Its properties are described on p. . There is also a `sixty-fifth line' in each half magnetic track. Its properties are described on p. .